Such apparatuses are described in French patent specifications numbers FR2717126A and FR2731952A.
In some types of vehicle air conditioning apparatus, the compressor is of the variable capacity, or variable output, type, that is to say the volume of fluid which it aspirates on each revolution is adjustable. Adjustment of the capacity of the compressor enables the flow of refrigerant fluid to be adapted to the requirements, or demand, for cooling, i.e. extraction of heat. It also enables variations the speed of the engine of the vehicle to be compensated for, having regard to the fact that engine speed also affects the running speed of the compressor.
Variable capacity compressors commercially available generally incorporate adjusting mechanisms which react in particular to values of the pressure of the fluid in the circuit. These regulating mechanisms have not given correct operation of an apparatus of the type envisaged by the invention in the heating mode. In particular, when the air conditioning apparatus is started up, the pressure prevailing in the refrigerant fluid circuit may be high, being for example of the order of ten bars. Having regard to this high pressure, the compressor sets its capacity at the maximum value so as to reduce the pressure. The capacity is then adjusted so as to bring the pressure to a set or demand value, for example three bars, so as to prevent any icing up of the evaporator.
In the case where the air conditioning apparatus also includes a heating loop, the pressure of the refrigerant fluid, which depends on atmospheric temperature, is for example 1.4 bar for a temperature of -20.degree. C. during starting up of the apparatus in the heating mode. The conventional method of regulating the output of the compressor would fix this pressure at its minimum value, giving a very small mass flow and very low heating efficiency.